Powered devices need to have a mechanism to supply power to the operative parts. Typically systems use a physical power cable to transfer energy over a distance. There has been a continuing need for systems that can transmit power efficiently over a distance without physical structures bridging the physical gap.
Systems and methods that supply power without electrical wiring are sometimes referred to as wireless energy transmission (WET). Wireless energy transmission greatly expands the types of applications for electrically powered devices. One such example is the field of implantable medical devices. Implantable medical devices typically require an internal power source able to supply adequate power for the reasonable lifetime of the device or an electrical cable that traverses the skin. Typically an internal power source (e.g. battery) is feasibly for only low power devices like sensors. Likewise, a transcutaneous power cable significantly affects quality of life (QoL), infection risk, and product life, among many drawbacks.
More recently there has been an emphasis on systems that supply power to an implanted device without using transcutaneous wiring. This is sometimes referred to as a Transcutaneous Energy Transfer System (TETS). Frequently energy transfer is accomplished using two magnetically coupled coils set up like a transformer so power is transferred magnetically across the skin. Conventional systems are relatively sensitive to variations in position and alignment of the coils. In order to provide constant and adequate power, the two coils need to be physically close together and well aligned.
Other solutions have focused on making one or both of the coils larger so the magnetic flux field is larger. This provides greater freedom of movement of the coils while ensuring efficient power transfer. This approach has several drawbacks. Because the components are implanted in the body, a larger receiving coil makes surgical placement more difficult and limits the implantation options. In cases where the transmitter is large relative to the receiver, it is likely the transmitter will be set up to generate a large field, then the receiver will have to adjust to keep its output in a reasonable range, and optimize for efficiency.
Enlarging the transmission coil has several drawbacks. The transmitter is effectively set up to blast out the maximum field, which necessitates careful power management on the receiver side. The transmitter is set up to operate passively, and the receiver is doing most of the control. Thus, this system can be complex to implement, especially in sensitive applications like implantable medical devices. A larger transmission coil decreases quality of life (QoL) because the patient must carry a larger, heavier coil. Moreover, larger coils inherently lead to greater complexity. The system inherently carries the risk of providing too much power to the internal load and/or battery. Higher power transfer can also lead to excessive heating, which risk damaging the internal organs and tissue.
In cases where the transmitter is smaller, such as a mobile wearable coil, transmitter efficiency is important. The system is highly sensitive to receiver coil position and orientation and may not provide adequate power transfer in all cases.
Another drawback with conventional wireless power transmission systems is high susceptibility to interference. Even minor changes to the field from external factors can significantly affect the transmitter and receiver. For example, if a metallic object is introduced into the system, it will affect the magnetic flux and in turn the system will not operate efficiently.